Given that ADB is enabled on these tablets, you can sideload Kindle app and any other app you wish.

But...if it can only be done using ADB, it's useless to the average consumer. On my Fire, I can check a little box to allow apps from other stores, and sideload the getjar store, which then allows me to sideload many other apps seamlessly. I did this for my fiance and my nephew, and both are happily downloading apps that aren't on the official Amazon app store. No Root or ADB or developer skills required. So yes, Amazon doesn't put other app stores on their devices by default, but they don't restrict their use in any way. Barnes and Noble restricts sideloading to the 1% (if that) that is capable of installing and using ADB.